The world population is the total number of humans alive on the planet Earth at a given time. According to estimates published by the United States Census Bureau, the world population in June 2005 was ~6,450,000,000. In line with population projections, this figure continues to grow at rates that are unprecedented prior to the 20th century. Approximately one fifth of all humans in the last six thousand years are currently alive. By some estimates, there are now one billion (thousand million) young people in the world between the ages of 15 and 24.

The United Nations Population Fund designated October 12, 1999 as the approximate day on which world population reached six billion. This was about 12 years after world population reached five billion, in 1987. The child that has been proclaimed by the United Nations Population Fund and welcomed by the U.N. Secretary-General Kofi Annan as the six billionth baby, was born on the designated day two minutes after midnight, not in India or China, as might be expected, but to Fatima Nevic and her husband Jasminko in Sarajevo, Bosnia.

The last 70 years of the 20th century saw the biggest increase in the world's population in human history. The following table shows when each billion milestone was met:

Including a few more estimates (beginning with 250 million around AD 950 and ending with 8 billion in 2027), the world population was doubled by the following years (doubling times in parentheses):

* AD 950 (650) 1600 (202) 1802 (125) 1927 (47) 1974 (50) 2027,

or (beginning with 375 million around year 1420):

* 1420 (300) 1720 (155) 1875 (86) 1961 (38) 1999.

Note how, until recently, each doubling has taken roughly half as long as the previous doubling.

The UN estimated in 2000 that the world's population was then growing at the rate of 1.2 percent (or 77 million people) per year. This represents a decrease in the growth rate from its level in 1990, mostly due to decreasing birth rates.

As of 2004, the world's population is increasing at a rate of 75 million people per year.

The future growth of population is difficult to predict. Birth rates are declining slightly on average, but vary greatly between developed countries (where birth rates are often at or below replacement levels) and developing countries. Death rates can change unexpectedly due to disease, wars and catastrophes, or advances in medicine. The UN itself has issued multiple projections of future world population, based on different assumptions. Over the last 10 years, the UN has consistently revised its world population projections downward.

Other projections of population growth predict that the world's population will eventually crest, though it is uncertain exactly when or how. In some scenarios, the population will crest as early as the mid-21st century at under 10 billion, due to gradually decreasing birth rates.

In less optimistic scenarios, disasters triggered by the growing population's demand for scarce resources will eventually lead to a sudden population crash, or even a Malthusian catastrophe.

In 1798, Thomas Malthus predicted that population growth would eventually outrun food supply, resulting in catastrophe. In 1968 Paul R. Ehrlich reignited this argument with his book The Population Bomb, which helped give the issue significant mindshare throughout the 1960s and 1970s. The dire predictions of Ehrlich and other neo-Malthusians were vigourously challenged by a number of economists, notably Julian Simon.

On the opposite end of the spectrum there are a number of doomsayers who argue that today's low fertility rates will have severe negative consequences: The Death of the West : How Dying Populations and Immigrant Invasions Imperil Our Country and Civilization, by Patrick Buchanan, The Empty Cradle: How Falling Birthrates Threaten World Prosperity, by Longman, and Fewer: How the New Demography of Depopulation Will Shape Our Future, by Wattenberg.

Child poverty has been linked to people having children before they have the means to care for them.

More recently, some scholars have put forward the Doomsday argument applying Bayesian probability to world population to argue that the end of humanity will come sooner than we usually think (toxic waste rather than food shortages).